Indirect reference using Atasm

[l12n]

How to store a value in memory at an address stored in a variable in Atasm? I tried various things to no avail:

 

player_ptr
    .ds 2

; set the value of player_ptr
...

; Try to modify the address stored in player_ptr
lda #$ff
sta *player_ptr     ; no-op?
sta (player_ptr)    ; the compiler complains of an Illegal indirect reference

 

[l12n]

OK, so after further research it appears the only way to do an indirect reference is by 1) using a X or Y register and 2) using a zero-page address

[Wrathchild]

[misread]

Edited January 18 by Wrathchild

[sanny]

5 hours ago, l12n said:

OK, so after further research it appears the only way to do an indirect reference is by 1) using a X or Y register and 2) using a zero-page address

Indeed

 

http://6502.org/tutorials/

[TGB1718]

7 hours ago, l12n said:

OK, so after further research it appears the only way to do an indirect reference is by 1) using a X or Y register and 2) using a zero-page address

You could use this type of method

Output screen shot shown

 

 

    ; pseudo pointer demo NOT using Page zero
    ; ptr is the pointer which will update each time the routine is called
player=$1ff0    ; where we want the data put, ensure we cross a boundary to check increment works ok

    org $600
    ldx #0
loop    jsr pointer
    inx
    cpx #[tabend-table] 
    bne loop
    rts

    ; this is where we update the ptr address

pointer inc ptr+1    ; increment lo byte
    bne ok
    inc ptr+2        ; increment hi byte if crossed a page
ok    lda table,x
ptr   sta player-1    ; ensure we get the first byte correct @$1ff0
    rts
    ; dummy data
table    .byte 1,2,3,4,5,6,7,8,9,0,1,2,3,4,5,6,7,8,9,0,1,2,3,4,5,6,7,8,9,0
tabend

 

[Rybags]

Not really an assembler limitation but the CPU itself.

 

I used AtAsm - firstly, though it's fairly close to Mac-65 there's at least a couple way better cross-assemblers in common use (ie MADS)

Next up - make sure you're using a later less buggy version, I remember having problems with an earlier one some time back.

[EddyFree]

On 1/17/2024 at 8:21 PM, l12n said:

How to store a value in memory at an address stored in a variable in Atasm? I tried various things to no avail:

 

player_ptr
    .ds 2

; set the value of player_ptr
...

; Try to modify the address stored in player_ptr
lda #$ff
sta *player_ptr     ; no-op?
sta (player_ptr)    ; the compiler complains of an Illegal indirect reference

 

Not sure what you are trying to do but you can always use self-modifying code as a pseudo-pointer.

 

Similar to this simple byte copy routine:

 

 

; set source = $1000 and
; destination = $2000
; in the $FFFF bytes below
LDA #$10
STA SOURCE+2
LDA #$20
STA DESTINATION+2

 

LDA #$00
STA SOURCE+1       ; +1 to skip over LDA OP code byte
STA DESTINATION+1  ; +1 to skip over STA OP code byte

 

LDX #$0

 

LOOP

 

; the following $FFFF bytes to be set by the code above
; and updated by the code below
SOURCE LDA $FFFF
DESTINATION STA $FFFF

 

; modify the $FFFF bytes above to point to next locations
INC SOURCE+1
BNE NOOVER1
INC SOURCE+2
NOOVER1

 

INC DESTINATION+1
BNE NOOVER2
INC DESTINATION+2
NOOVER2

 

INX

 

BNE LOOP

 

[Qwe]

 

If I remember correctly, indirect addressing on the 6502 is only possible if the variable that acts as a pointer is on page 0.

[Wrathchild]

8 hours ago, Qwe said:

indirect addressing on the 6502 is only possible if the variable that acts as a pointer is on page 0.

In terms of load/store, yes. Indexed Indirect (using X) and Indirect Indexed (using Y) are both 2 byte instructions, the second being the zero-page address.

Indirect JMP however is a 3 byte instruction using a word address, a pointer to the address holding the word address loaded to the PC.